Mark Lynas is the author of "Six Degrees: Our Future on a Hotter Planet" (HarperPerennial, £8.99), winner of the 2008 Royal Society Science Book Prize.
Biofuels are an obvious solution: replace "mineral" petrol and diesel from fossil reserves with "biological" fuels extracted from plants and the result will be no net addition of CO2 to the atmosphere.
Or will it? Perhaps the strongest argument against biofuels is that they simply replace one ecological problem with another. A large-scale shift towards biofuels - extracting fuel from the biosphere rather than underground - can only worsen the human agricultural pressure on ecosystems, as we shift from producing not just food but also fuel from increasingly scarce cultivable land.
But there are other less visible problems too. Most farmers apply nitrogen-based fertilisers to their crops to stimulate production. Most of this nitrogen isn't captured by the plants, but runs off into rivers and lakes, causing algal blooms which kill fish and deplete oxygen levels. Whole areas of the ocean are now classified as "dead zones", because of this agricultural runoff.
But the ecological concerns raised by biofuels run even deeper than this. With more than six billion people on the planet, humanity has already run short of agricultural land for food production, and the conversion of virgin forests and grasslands into farmland monoculture can only worsen the current extinction crisis.
Biofuels supporters frequently advocate the use of plants like the oilseed-producing drought-tolerant shrub jatropha, which they argue can be grown in "marginal" areas in poorer countries without reducing food production. However, "marginal" areas are often precisely the places where a semblance of biodiversity still clings on.
In addition, if crops like jatropha become successful, they will doubtless be expanded into food-producing areas and forests alike: unless strict laws are in place, economic incentives will always trump humanitarian or ecological concerns. Similarly, so-called "second generation" biofuels are also touted as a radical improvement on current fuel production from food crops. If ethanol is brewed from crop waste or wood, the argument goes, biofuels production can be ramped up without driving up food prices and starving the poor. But if this "cellulosic ethanol" were ever to take off in a big way, it might present an even greater threat than today's generation of biofuels. It is likely that entire forests would be liquefied in order to produce petrol and diesel for motorists.
That is not to say that all biofuels are bad. Burning old chip fat in car engines is beneficial, but only on a tiny scale.
The best way to reduce emissions from vehicles is not to find new sources of liquid fuels, but to shift rapidly to the production of electric cars and trucks, which can plug into the grid to recharge. This electricity in turn must come from wholly renewable sources, which means wind, solar and wave or tidal power.
Over the century ahead humanity has to learn how to supply its energy needs in ways which do not destroy the capacity of the planet to support life. Neither biofuels nor fossil fuels meet this test - but luckily there are plenty of energy sources that do.